As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can only be read by a user. The optical storage disc may also be a writable type, where information may be stored by a user. For writing information in the storage space of a writable optical storage disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand light generating means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam. Since the technology of optical discs in general, and the way in which information can be stored in an optical disc, is commonly known, it is not necessary here to describe this technology in great detail. For understanding the present invention, it is sufficient to mention that the laser beam is modulated such as to cause a pattern of locations where properties of the disc material have changed, such pattern corresponding to coded information. Generally, the laser drive signal is a digital signal. During recording an optical disc the laser driver controls the laser by a drive current. This drive current, and consequently the light emitted by the laser, follows a desired pattern. In fact the drive current is modulated in amplitude as a function of time. For various reasons, e.g. because of a data erase function, the drive current may (but not necessarily) have more than two values. The way in which the laser is controlled for generating the desired pattern of light pulses is generally termed a write-strategy and is generally performed by a Write Strategy Generator.
Generally a laser starts to radiate light if the electrical power supplied to it is above a certain threshold. The supplied power corresponding to this threshold will be further denoted as “threshold power”. In order to be able to write, read, or erase information to/from the optical disk the laser needs a certain amount of power on top of the threshold power. This is generally denoted as “delta power”. The power supplied to the laser is controlled by said Write Strategy Generator.
The threshold of a laser is not always the same. It may shift due to a change in temperature. Also by ageing of the laser the threshold can change. Also the amount of delta power may shift due to various causes. Therefore it is generally needed to control the threshold power and the delta power of the laser. Preferably this is performed automatically, e.g. by applying one or more feedback loops.
The Write Strategy Generator delivers data levels which drive the laser via a digital-to-analog converter. Since the data levels must be relatively accurate quite a lot of bits (e.g. 16 bits) are used. In the case of a laser driver the digital-to-analog converter is preferably divided in two parts, further to be denoted by a threshold part and a delta part. Then also the data levels are divided in two parts (e.g. two parts of 8 bits). The gains of the threshold and delta parts are controlled by a so-called threshold reference current and a delta reference current, respectively. This enables the separate control of the threshold power and the delta power of the laser.